1. Field of the Invention
The present invention pertains generally to baghouses for separating dust from gas streams.
More particularly, the present invention pertains to means for controlling the re-inflation of bag filters in a baghouse following reverse gas flow cleaning.
2. State of the Prior Art
Many kinds of dust produced in industrial processes can be separated from gas streams by fabric filters made of natural or synthetic fibers. A prevalent dust filtration technique utilizes a plurality of tubular bag-like fabric filters, referred to as bag filters, mounted in an enclosed structure called a baghouse. The bag filters are usually mounted vertically within the baghouse, and are open at their bottom ends. In a typical baghouse arrangement, streams of a dust-laden gas to be filtered are passed into the interiors of the bag filters through the open bottom ends. Dust from the gas streams is retained on the inside surfaces and in the interstices of the fabric of the bag filters, and dust-free gas is drawn through the fabric into the interior of the baghouse external to the bag filters. A pressure difference (maintained by conventional means) between the interior of the baghouse and a clean gas plenum causes dust-free gas to be drawn into the clean gas plenum for passage either to a region where the gas is to be used or further processed, or to a stack for release to the atmosphere.
The dust that collects on the inner surfaces of a bag filter forms a cake, referred to as the dust cake, which acts as a filter medium for removing other dust from the gas stream. However, the dust cake increases the impedence of the bag filter to the passage of gas therethrough. Consequently, the bag filters in a baghouse must be cleaned periodically to remove a major portion of the dust cake therefrom so that gas flow therethrough can be maintained at an acceptable rate.
A method for removing the dust cake from the inner surfaces of the bag filters in a baghouse involves periodically stopping the flow of dust-laden gas into the baghouse and closing an opening to the clean gas plenum, and then directing a flow of dust-free gas from the clean gas plenum back into the baghouse for passage through the bag filters in the opposite direction. This method is called reverse gas flow cleaning.
Dust-laden gas flow in the normal direction through the filter fabric (i.e., in the direction of dust cake formation) tends to press the dust cake against the fabric. Stopping the flow of dust-laden gas in the normal direction serves to release the dust cake from the fabric; and directing the burst of dust-free gas in the reverse direction through the fabric serves to push the dust cake away from the fabric. The dust cake thus removed from the bag filters falls into a hopper for collection and eventual removal from the baghouse. A discussion of particular baghouse designs, applications and filter cleaning techniques known to the prior art can be found in an article by Milton N. Kraus entitled "Baghouses: Separating and Collecting Industrial Dusts" published in Chemical Engineering, Apr. 9, 1979, pages 94-106.
During bag filter cleaning by the reverse gas flow method, the bag filters in a baghouse tend to collapse inward or deflate due to the greater pressure on the outside surfaces than on the inside surfaces of the bag filters. Upon re-establishment of dust-laden gas flow in the normal direction following completion of the reverse gas flow cleaning of the fabric, the bag filters are re-inflated to their normal configurations. Rapid and uncontrolled re-inflation of the bag filters, however, can have a deleterious effect on the filter fabric. Sudden resumption of the gas flow in the normal direction can tear the fabric, a phenomenon known as "bag popping".
Various valving techniques have been developed in the prior art for controlling gas flow in baghouse operations. Examples of such techniques are described in U.S. Pat. Nos. 3,057,137 to D. B. Perlis et al.; 3,396,516 to W. E. Ballard; 3,521,430 to H. D. Vanderlip et al.; 3,540,193 to J. Pausch; 3,898,062 to P. B. Slakey; 3,926,595 to O. K. Bockman; 3,945,400 to P. B. Slakey; and 3,963,467 to D. W. Rolschau. Until the present invention, however, a reliable and relatively inexpensive valving technique for precisely controlling the re-inflation rate of bag filters in a baghouse had not been known, which could readily take into account such variables as the dimensions and material of the bag filters, the nature and flow rate of the gas stream, the nature and concentration of the dust in the gas stream, and the rapidity of change of any of these variables.